1. Field of the Invention
The present invention relates generally to audio devices which incorporate microphones for sensing sound, and more particularly, to an apparatus and method for measuring the relative frequency response of two or more microphones provided within such an audio device to facilitate a fixed calibration compensation system.
2. Description of the Relevant Art
A microphone is one of the key components in many audio products, including those used for telecommunications. A microphone is a transducer that converts acoustic (sound) energy into electrical energy. It is known to employ speech enhancement algorithms and/or noise reduction algorithms within such products to process incoming signals from a microphone to enhance the performance of such products in acoustically challenging environments, e.g., in the presence of unwanted background noise.
Until recently, the majority of consumer electronics used only a single microphone. With rapid advances in high speed digital signal processors, speech enhancement algorithms and noise reduction algorithms have started using two or more microphones to exploit the spatial diversity that exists between such microphones. In certain scenarios, these multiple-microphone-based algorithms can provide sound quality far superior to single microphone implementations. Today, speakerphones, conference phones used to conduct telephone conference calls in an office conference room, and even Bluetooth® telephone earpieces, often employ two or more microphones to sense surrounding sounds. In the case of conference phones, the use of multiple microphones, together with digital signal processing, helps to ensure that all speakers are detected while allowing the audio device to focus on the active speaker at any given point in time. The use of multiple microphones is also key to achieving echo cancellation and suppression of unwanted background noise signals.
However, the improved performance of these multiple-microphone-based algorithms introduces many new problems. For example, it has been found that audio devices that use multiple microphones to achieve speech enhancement and/or noise reduction perform poorly if the frequency responses of such microphones are not well matched. If the microphones used within a particular audio device are well matched to each other, then the relative frequency response will be approximately zero over the frequency band of interest, both in terms of relative magnitude and relative phase.
Both the magnitude and the phase responses of the microphones are critical to successful implementation of modern algorithms for speech enhancement and noise reduction. In some cases, it is not necessary to know the individual phase response of a particular microphone; instead, the relative phase response between any two microphones is sufficient information for most of the algorithms to work properly. Accordingly, if it were possible to determine the relative frequency response, including the relative magnitude and the relative phase responses, between two microphones, then such information can be used to compensate for differences between such microphones. Unlike magnitude response measurement, relative phase response measurement between two microphones is an extremely difficult problem. At higher frequencies, even a small positional variation in the measurement set-up (even of a few millimeters) can drastically affect the phase measurement results. In order to comply with ITU-T wide band mode standards, the relative frequency response must be considered over at least the range of 100 Hz to 7000 Hz.
Various compensation techniques have been used in the past to account for microphones that differ in relative frequency response. Self-calibration is a technique used to adjusting the parameters of a compensation system using an excitation signal that is usually present during the normal mode of operation of the audio device. One example of this self-calibration technique is disclosed within Patent Application Publication No. US 2004/0165735, published Aug. 26, 2004. On-line calibration is a second technique adjusting the parameters of a compensation system, wherein the parameters of the calibrating system are adaptively updated during the normal mode of operation. An example of this calibration technique is disclosed within U.S. Pat. No. 6,914,989, issued to Janse, et al., on Jul. 5, 2005.
A third technique used to adjust the parameters of a compensation system is known as “fixed calibration”; a fixed calibration technique refers to measuring the relative frequency response between a pair of microphones using an off-line process, and then initializing a fixed set of calibration parameters based on the measurement. One of the difficulties of effectively implementing a fixed calibration compensation technique is accurately determining the relative frequency response as between two microphones. To accurately determine such relative frequency response as between two microphones within a frequency band of interest, one must know both the differences in the magnitude response of the two microphones as well as the differences in phase response of the two microphones.
In addition, within some audio devices, the audio path to the first microphone and the audio path to the second microphone differ from each other. Thus, even if the two microphones were themselves perfectly matched to each other, the difference in the respective audio paths leading to the first and second microphones may result in a relative frequency response that needs to be compensated. In some audio products, the microphones are mounted deep inside the outer housing of the product. The frequency response of the installed microphones can sometimes drastically differ from the free standing frequency response of each such microphone. Accordingly, the mechanical design of the microphone housing, along with the acoustic path inside the product, can greatly affect the overall frequency response of the acquired signal that will be used for further signal processing.
Some of the factors that will affect the overall frequency response are: the acoustic tube length from the microphone hole in an audio product to the port in the microphone capsule; multi-path acoustic leakage; resonant cavities; and improper microphone booting. Hence, for those products in which microphones are embedded deep within the outer housing, it is important to measure the overall frequency response that encompasses both the microphone itself and the acoustic path to the microphone, rather than merely measuring the frequency response of the free standing microphone. The measurement logistics are further complicated by the fact that audio products using the same types of microphones come in various shapes and sizes. The accessibility of microphone holes further complicates the measurement process too.
Adding to the complexity of relative frequency response, there are a variety of different types of microphones in current use, including electret condenser microphones (or “ECMs”) and micro electro-mechanical system (so-called “MEMS”) microphones. A cylindrically-shaped electret (ECM) microphone might have typical dimensions of 9.5 mm in diameter×6.3 mm in height. In contrast, a cuboidal micro electro-mechanical system (MEMS) microphone would typically have much smaller dimensions, on the order of 3.76 mm in length, ×3.0 mm in width, ×1.1 mm in height. A test apparatus used to detect the relative frequency response of microphones would need to be capable of accommodating at least both such types of microphones.
One known technique for measuring the relative frequency response as between two microphones is to position both microphones within a test chamber, equidistant from a loudspeaker, and to alternately measure the response of each microphone to an excitation signal issued by the loudspeaker. However, the two microphones are positioned at two different points in space within the test chamber, each having its own unique propagation path. As already noted above, differences in the propagation paths for two microphones can change the effective relative frequency response of such microphones. It is therefore important to minimize any differences in the propagation paths to the microphones under test when designing a measurement set up in order to obtain accurate results.
Theoretically, one could maintain the propagation path to the two microphones substantially constant by first positioning the first microphone at a given point in the test chamber, measuring the frequency response of the first microphone, then removing the first microphone, replacing it with the second microphone at the same given point, and measuring the frequency response of the second microphone. However, even small changes in a physical set up between two successive measurements can alter the acoustic field. For example, the acoustic field can change when the first microphone is manually replaced by the second microphone in the measurement set up. A small change in the apparatus position between the two measurements can also introduce different diffraction patterns, thereby affecting the acoustic field at the sensing point.
Accordingly, it is an object of the present invention to provide a test set-up apparatus and method for measuring the phase and magnitude differences between first and second microphones for use in a fixed calibration system in a manner that is non-destructive to the microphones under test, and is reliable and repeatable, even when the test set-up is disassembled and reassembled several times.
Another object of the present invention is to provide such a test set-up and method which provides a smooth response to measured magnitude and phase without extreme variations, especially at higher frequencies.
Still another object of the present invention is to provide such a test set-up and method capable of handling various microphone types and shapes, including both ECM-style and MEMS-style microphones.
A further object of the present invention is to provide such a test set-up and method capable of measuring the relative frequency response of microphones over at least the frequency range of 100 Hz to 8,000 Hz.
A still further object of the present invention is to provide such a test set-up and method that is relatively simple to prepare and conduct, allowing such measurements to be completed in less than 30 minutes.
Yet a further object of the present invention is to provide such a test set-up and method which minimizes any changes in the acoustic field, while maintaining a consistent propagation path, when alternating between measurements of frequency response of first and second microphones.
Another object of the present invention is to provide such a test set-up and method capable of performing such relative frequency response measurements even when two or more microphones are mounted deep inside an outer housing of an audio product.
These and other objects of the invention will become more apparent to those skilled in the art as the description of the present invention proceeds.